It is well known that nickel based superalloys are extensively employed in high performance environments. Such alloys have been used extensively in jet engines and in gas turbines where they must retain high strength and other desirable physical properties at elevated temperatures of 1000.degree. F. or more.
For most such alloys the attainment of such a combination of properties is at least partly due to the presence therein of a .gamma.' precipitate or a .gamma." precipitate sometimes referred to as strengthening precipitates.
More detailed characteristics of the phase chemistry of precipitate phases are given in "Phase Chemistries in Precipitation-Strengthening Superalloy" by E. L. Hall, Y. M. Kouh, and K. M. Chang [Proceedings of 41st. Annual Meeting of Electron Microscopy Society of America, Aug. 1983 (p. 248)].
The following U.S. patents disclose various nickel-base alloy compositions: U.S. Pat Nos. 2,570,193; 2,621,122; 3,046,108; 3,061,426; 3,151,981; 3,166,412; 3,322,534; 3,343,950; 3,575,734; 3,576,681; 4,207,098 and 4,336,312. The aforementioned patents are representative of the many alloying situations reported to date in which many of the same elements are combined to achieve distinctly different functional relationships between the elements such that phases providing the alloy system with different physical and mechanical characteristics are formed. Nevertheless, despite the large amount of data available concerning the nickel-base alloys, it is still not possible for workers in the art to predict with any degree of accuracy the physical and mechanical properties that will be displayed by certain concentrations of known elements used in combination to form such alloys even though such combination may fall within broad, generalized teachings in the art, particularly when the alloys are processed using heat treatments different from those previously employed.
A problem which has been recognized to a greater and greater degree with many such nickel-base superalloys is that they are subject to formation of cracks or incipient cracks, either in fabrication or in use, and that the cracks can actually propagate or grow while under stress as during use of the alloys in such structures as gas turbines and jet engines. The propagation or enlargement of cracks can lead to fracture of parts formed of such superalloys or other failure. The consequence of the failure of the moving mechanical part due to crack formation and propagation is well understood. In jet engines it can be particularly hazardous and even catastrophic.
However, what has been poorly understood until recent studies were conducted was that the formation and the propagation of cracks in structures formed of superalloys is not a monolithic phenomena in which all cracks are formed and propagated by the same mechanism and at the same rate according to the same criteria. By contrast the complexity of the crack generation and propagation and of the crack phenomena generally and the interrelation of such propagation with the manner in which stress is applied is a subject on which important new information has been gathered in recent years. The period during which stress is applied to a member to initiate or to propagate a crack, the intensity of the stress applied, the rate of application and of removal of stress to and from the member, and the schedule of this application, was not well understood in the industry until a study was conducted under contract to the National Aeronautics and Space Administration. This study is reported to a technical report identified as NASA CR165123 issued from the National Aeronautics and Space Administration in Aug. 1980 and identified as "Evaluation of the Cyclic Behavior of Aircraft Turbine Disk Alloys" Part II, Final Report, by B.A. Cowles, J.R. Warren and F.K. Hauke, prepared for the National Aeronautics and Space Administration, NASA Lewis Research Center Contrast NAS3-21379.
A principal unique finding of the NASA sponsored study was that the rate of propagation based on fatigue phenomena or in other words the rate of fatigue crack propagation (FCP) was not uniform for all stresses applied nor to all manners of applications of stress. More importantly, the finding was that fatigue crack propagation actually varied with the frequency of the application of stress to the member where the stress was applied in a manner to enlarge the crack. More surprising still, was the finding from the NASA sponsored study that the application of stress of lower frequencies rather than at the higher frequencies previously employed in studies actually increased the rate of crack propagation. In other words the NASA study revealed that there was a time dependence in fatigue crack propagation. Further the time dependence of fatigue crack propagation was found to depend not on frequency alone but also on the time during which a member was held under stress or a so-called hold time. Following the discovery of this unusual and unexpected phenomena of increased fatigue crack propagation and lower stress frequencies there was some belief in the industry that this newly discovered phenomena represented an ultimate limitation on the ability of the nickel based superalloys to be employed in the stress bearing parts of the turbines and aircraft engines and that all design effort had to be directed to design around this problem.
However, it has now been discovered that it is feasible to construct parts of nickel based superalloys for use at high stress in turbines and aircraft engines with greatly reduced crack propagation rates.
The development of new superalloy processing and compositions has now focused on the fatigue property and has addressed in particular the time dependence of crack growth.
Crack growth, i.e., the crack propagation rate, in high-strength alloy bodies is known to depend upon the applied stress (.sigma.) as well as the crack length (a). These two factors are combined by fracture mechanics to form one single crack growth driving force; namely, stress intensity K, which is proportional to .sigma..sqroot.a. Under the fatigue condition, the stress intensity in a fatigue cycle represents the maximum variation of cyclic stress intensity (.DELTA.K), i.e., the difference between K.sub.max and K.sub.min. At moderate temperatures, crack growth is determined primarily by the cyclic stress intensity (.DELTA.K) until the static fracture toughness K.sub.IC is reached. Crack growth rate is expressed mathematically as da/dN.varies.(.DELTA.K).sup.n. N represents the number of cycles and n is a constant, which is between 2 and 4. The cyclic frequency and the shape of the waveform are the important parameters determining the crack growth rate. For a given cyclic stress intensity, a slower cyclic frequency can result in a faster crack growth rate. This undesirable time-dependent behavior of fatigue crack propagation can occur in most existing high strength superalloys.
The most undesirable time-dependent crack-growth behavior has been found to occur when a hold time is superimposed on a sine wave variation in stress. In such case a test sample may be subjected to stress in a sine wave pattern but when the sample is at maximum stress the stress is held constant for a hold time. When the hold time is completed the sine wave application of stress is resumed. According to this hold time pattern the stress is held for a designated hold time each time the stress reaches a maximum in following the normal sine curve. This hold time pattern of application of stress is a separate criteria for studying crack growth. This type of hold time pattern was used in the NASA study referred to above.
A main design objective is to make the value of da/dN as small and as free of time dependency as possible.